The present invention relates generally to control systems and more particularly to a control system of the direct ignition type.
In the past, control systems for fuel burners, such as gas furnaces and the like, typically employed a thermostat to open a fuel valve on demand for heat and also typically employed means for automatically igniting the fuel emitted from the burner. Ignition was sometimes achieved by a small continuously burning pilot flame or by using a spark or arc igniter to provide a spark in the vicinity of the burner at about the same time or soon after the fuel valve was opened.
An improvement in control systems for fuel burners is illustrated in U.S. Pat. No. 3,734,676. In this patented control system, an igniter and a controlled switching device are effective when gated to connect the igniter to a source of electrical energy for providing pulses of electrical energy to the igniter. This patented control system has a gate circuit including a capacitance for providing gate signals to the controlled switching device. These gate signals are provided so long as the capacitance has a charge below a specified level and the capacitance is initially provided with a charge above that specified level. The capacitance charge is reduced with time so as to gate the controlled switching device to operatively connect the igniter to a source of electrical energy only after a predetermined period of energization of the control system thereby to effect preignition purging of the burner area to insure that there is not an over abundance of fuel when ignition occurs.
Control systems of the type illustrated in the aforementioned U.S. Patent typically operate on a 120 volt alternating current supply or employ a step-up transformer from the commonly encountered low voltage thermostat systems which typically operate around 24 volts alternating current. U.S. Pat. No. 3,920.376 and a copending application Ser. No. 504,252 filed Sept. 9, 1974, now U.S. Pat. No. 3,938,941, also disclose control systems which are amenable to direct energization by the aforementioned low voltage thermostat systems.